1. Field of the Invention
This invention relates to a television signal scanning line converting apparatus wherein, on the transmitting side, a progressive scanning signal is converted to an interlaced scanning signal which is transmitted and, on the receiving side, the interlaced scanning signal is converted to a progressive scanning signal.
2. Background of the Prior Art
In an NTSC system television broadcast, there is adopted an interlaced scanning system in which, by dividing one picture into a first field and second field which are transmitted, the band width is saved and a highly efficient transmission is made. However, as the television receiving apparatus becomes high in the luminance and large in the picture, a line flicker becomes conspicuous. Therefore, in the television receiving apparatus or the like, there may be adopted a scanning line converting apparatus wherein an interlaced scanning signal is converted to a progressive scanning signal by making an interpolation by using an image memory in order to reduce the line flicker and to improve the resolution.
FIG. 5 (PRIOR ART) is a block diagram showing a related art of such a television signal scanning line converting apparatus. FIG. 6 (PRIOR ART) is an explanatory view for explaining the operation of the prior art by taking the time of a field unit in the horizontal direction and taking the vertical direction of the drawing in the vertical direction. In FIG. 6, the white circle represents a scanning line based on a television signal transmitted by an interlaced scanning system and the black circle represents an interpolating scanning line obtained by an interpolation.
An interlaced signal input through an input terminal 1 is given to an intra-field interpolating filter 2 and inter-field interpolating filter 3. As shown by an interpolating scanning line 10 in FIG. 6, the intra-field interpolating filter 2 obtains an interpolating signal from the upper and lower scanning lines within the same field and outputs this interpolating signal to a switching circuit 4 as an interpolating signal for a moving picture. On the other hand, as shown by an interpolating scanning line 11 in FIG. 6, the inter-field interpolating filter 3 obtains an interpolating signal from a scanning line of a front field and outputs this interpolating signal to the switching circuit 4 as an interpolating signal for a still picture. The interlaced signal from the input terminal 1 is given also to a movement detecting circuit 5 which judges whether the transmitted interlaced signal is a moving picture or still picture and gives the judging signal to the switching circuit.
Now, in case the interlaced scanning signal from the input terminal is a still picture, the switching circuit 4 will output the output of the inter-field interpolating filter 3 as an interpolating signal on the basis of the judging signal. In this case, the now transmitted scanning line and the scanning line of the front field will be simultaneously used and all effective scanning lines (480 lines in the NTSC system) will be used for the process. Therefore, in the NTSC system still picture, a vertical resolution of 480 [TVL/PH]([TV lines/picture height]) can be obtained.
On the other hand, in the moving picture, as the picture pattern is different between the front and rear fields, if it is processed between the fields as in the interpolating scanning line 11 in FIG. 6, by the lag between the front and rear fields, the image will be doubled and the displayed picture quality will be broken. Therefore, in the moving picture, as shown by the interpolating scanning line 10 in FIG. 6, an intra-field interpolation is adopted and, by the judging signal from the movement detecting circuit 5, the switching circuit 4 outputs the output of the intra-field interpolating fiter 2 as an interpolating signal. Thereby, the interpolating signal will be produced from only the signal within the same field and the picture quality will not be broken.
The interpolating signal from the switching circuit 4 is reduced in time by 1/2 by a time compressing circuit 7 and is given to the terminal b of a switch 8. An interlaced scanning signal from the input terminal 1 is reduced in time by 1/2 by a time compressing circuit 6a and is given to the terminal a of the switch 8. The switch 8 selectively switches the terminals a and b for each scanning line by a horizontal period switching signal. That is to say, the scanning line not transmitted by the interlaced scanning in a predetermined field is interpolated by selecting the output of the time compressing circuit 7 with the switch 8. Thus, a progressive scanning signal is output from the output end of the switch 8.
Now, in the case of the NTSC system, the number of effective scanning lines within one field is 240 lines. Therefore, in the case of the interpolation by the scanning lines within the field, a vertical resolution above 240 [TVL/PH] will not be able to be obtained in principle. That is to say, in the moving picture, only a vertical resolution of 240 [TVL/PH] can be obtained. On the other hand, in the still picture, as described above,
a vertical resolution of 480 [TVL/PH] can be obtained by the inter-field interpolation.
However, if the vertical band is limited to 240 [TVL/PH] on the transmitting side, the picture quality will become obscure. Therefore, on the transmitting side, a band wider than 240 [TVL/PH] can not help being transmitted. That is to say, in the moving picture, as a signal in a wide band is input, a turning distortion will be generated, further only a resolution 1/2 that of the still picture will be obtained and the picture will be obscure. If the interpolating method is switched in conformity with the motion of the picture pattern, at the time of a still picture, a sharp image will be made but, at the time of a moving picture, an obscure image having a turning distortion will be made. There has been a problem that, by the resolution difference between the time of the still picture and the time of the moving picture, the video image will be unnatural in the sight.